Electrical actuators are used in a variety of systems to control the position of some mechanical device, and may be of either the rotary or linear type. In HVAC systems for example these mechanical devices may be a valve for controlling flow of fuel or hot water, or a damper for controlling air flow. Rotary actuators of the type involved here have their output element driven by a reversible electric motor through a gear train. Linear actuators may be either directly driven as by a solenoid or operate through a rack and a motor-driven pinion.
Regardless of the particular type of actuator, the motor or solenoid will have a first pair of power terminals to which is applied electric power for driving the motor in a first direction, and a second pair of power terminals for driving the motor in the opposite direction. The range of movement, or stroke range, for the output element is controlled at least one end by a limit switch through which the electrical power for the motor passes. A feature such as a cam or lever carried by the output element mechanically opens a limit switch when the output element reaches the limit position controlled by that limit switch.
It is most efficient from a standpoint of accurate control and minimum wear on the actuator and controlled device that the actuator hold the position to which its output element has been driven when power is removed from the drive motor or solenoid. Therefore, actuators are usually provided with a brake which is applied whenever power is removed from the motor, whether the power is removed by the control element of the system or by the limit switch. This brake may be applied by electric power supplied from the control circuitry of the actuator, or may be released by electric power and held set by a spring.
For certain types of mechanical devices, it is important if electrical power to the system is lost, that the actuator output element reset to what will be called a return position at one end of the stroke range. For example, if the actuator is controlling the position of a fuel valve, it is vital that the valve be promptly closed if electric power to the burner control device is lost. When this function is needed, an actuator may be provided with a return spring which drives the output element back toward its return position if power to the actuator control circuitry is lost. The brake for such an actuator will be of the type which is set by electric power and releases when power is not available to the control circuitry. When power is lost, the brake is released and the return spring acts to reset the output element to its return position. In a rotary actuator, such a spring is powerful enough to drive the drive motor in its reverse direction even through a relatively high ratio gear train. A mechanical stop is required to block the return spring from driving the output element past its return position. Such return springs are constantly wound and unwound as the motor drives the output element back and forth, so the motor must generate sufficient torque or force through its drive coupling to overcome the opposing spring load.
Certain types of actuator loads, which will be referred to hereafter as dynamic loads, not only oppose the torque provided by the actuator but also tend to drive the actuator's output element in the opposite direction when power is removed from the actuator. The return spring described above and used on some actuators is one example of such a dynamic load. But there are other types as well, such as an air damper's heavy shutters which tend through gravitational force to close and drive the actuator output element toward its return position when power is removed from the actuator which opened them.
In dynamic load situations, it is possible that when the limit position of the output element is reached and the limit switch opens, the dynamic load will cause the output element to return or reverse a sufficient amount to cause the limit switch to reclose before the brake sets. In this situation, the actuator motor then applies torque or force to the output element to again drive the element to the limit position and again open the limit switch. The actuator enters an oscillation or dither mode with the limit switch being constantly opened and closed. This situation has arisen where a rotary actuator has been redesigned to provide higher return torque in the return spring. Such a dither mode results in excessive wear on various parts of the actuator such as the motor, bearings, gear train if one is present, and limit switch, and premature failure of the actuator. At the same time, the clatter of an actuator operating in this mode is annoying and reduces the user's confidence, with some justification, in the reliability of the actuator itself.
It is possible to deal with this artifact of actuator operation by special design of the actuator's control circuitry which senses this dither and either removes power from the actuator or briefly overrides the limit switch, but this is an expensive and complex solution to the problem and also has the potential for damaging the driven mechanical device if the limit switch override feature should fail to function as designed. It is also possible to employ some type of special limit switch which has hysteresis in its operation, closing at a different position of the output element than it opens so that the small return of the output element when the limit switch opens does not cause the limit switch to reclose. Such a switch may be relatively complex and expensive and hence not desirable.